The use of lasers to mark patterns on a variety of materials and products (hereinafter simply referred to as workpieces) is well known. When the mark is to take the form of upsetting or ablating the material of the workpiece, the energy density of radiation emitted from a laser is often too low to mark directly. For this reason the laser beam has often been focused or imaged using a lens system to reduce the size of the interaction location on the workpiece and hence increase the energy density of the radiation at such location.
Two common laser marking methods are known as focused spot marking and mask imaging marking.
In focused spot marking a low pulse energy, high repetition rate, pulsed laser is used to mark a series of dots sequentially on the workpiece so that the selected pattern is eventually built up from these dots. In order to achieve enough energy density to form a mark, the laser beam is usually focused by a lens to a spot on the workpiece surface. The size of the spot can be adjusted by changing the focal length of the lens or by moving the workpiece away from the exact focal point of the lens in which case the size of the spot increases. For some materials, a continuous (non-pulsed) laser is also used for focused spot marking, in which case the mark is made up of continuous lines.
The mask imaging marking method uses a laser with a higher pulse energy and a beam profile that can mark all the information of the pattern on the workpiece with a single pulse. A stencil mask is used to transmit only the light containing the desired pattern information. The pattern is usually in a substantially solid format, rather than being made up of a large number of individual dots. In many cases, the energy density of the light forming the patterned array is not high enough to mark the product directly, so it is imaged using a lens system to reduce its size by a demagnification factor. Since the interaction area is then reduced by an amount equal to the square of the demagnification factor, the energy density at the workpiece surface is increased by the same amount. However, the size of the overall pattern formed on the workpiece is decreased by this procedure, which is often a disadvantage. Thus, a laser with a given pulse energy can be used to mark on a given material a pattern of a size that is determined by the laser pulse energy and the energy density required to mark that particular material. To achieve a larger pattern on the same material, the laser pulse energy must be increased, and this is usually difficult or expensive in practice.
On the other hand, this mask imaging technique has a number of advantages over the focused spot matrix marking technique. First, since the whole pattern is formed simultaneously, e.g. with a single pulse, the rate of marking of workpieces is often much faster and the pattern can be applied to a rapidly moving series of workpieces. Also, the characters or other symbols forming the selected patterns can be made relatively bold and hence more readily recognisable by the human eye, since they are formed from almost solid elements rather than dots. However, as indicated above, a major disadvantage of the mask imaging technique is that it requires a high pulse energy laser which is bulky and often expensive compared to a low pulse energy, higher repetition rate laser of the same average power, that is used to carry out the focused spot technique.